There have been attempts to protect the interior of the vehicle from an incursion, utilizing an acoustic detector responsive to sound waves and converting those sound waves in the interior of a vehicle into an electrical measurement signal.
The electrical measurement signal can be amplified by an amplifier circuit and can activate an alarm device. The sound detector can be an electroacoustic element or transducer which is responsive to sound waves in the audible and/or ultrasonic ranges and which transform the sound waves into electrical oscillation.
The sound waves can be waves which are transmissible through gaseous media, air in the case of a vehicle interior, or so-called body waves, namely, waves which are transmitted through the vehicle chassis or other solid bodies forming the vehicle interior.
The electrical oscillations are processed as measurement signals, usually with the aid of analog amplification. The alarm device can be an acoustic and/or optical signal generator. For example, the alarm device can be a horn or other sound generator commonly provided for the vehicle and/or an illumination device or lamp forming part of the vehicle, for example the headlight. The alarm can also be a signal-generator independent from the vehicle electrical circuitry. The alarm can produce a radio signal which can be picked up by a radio wave receiver remote from the vehicle.
Utilizing such means, an incursion into the interior of a vehicle can set off an alarm. The incursion may be the breaking of a vehicle window, an impact upon the window upon insertion of a mechanical device for unauthorized activation of the vehicle lock mechanism, or simply the entry of an unauthorized person through a door of the vehicle, by opening of the door, or an incursion into the vehicle space in some other manner.
Devices hitherto used for this purpose can employ sound transmitters which can permanently generate ultrasonic waves which are reflected and detected, the detected waves being evaluated by Doppler methods. In these methods, frequency and phase comparison can be effected between generated and received sound waves. If an object or a person moves with a component of the movement in the radiation direction of a sound generator or receiver, the reflected sound waves will show a frequency shift which is proportional to the velocity of the moving component.
By a comparison of the transmitted sound waves with the received sound waves, a measurement signal can be produced whose magnitude depends upon the detected velocity of the object or person. When the measurement signal exceeds a predetermined threshold, the alarm is activated. These systems operate generally with analog electronics and it may be noted that conventional analog electronics operate with relatively poor precision so that a high threshold is required to prevent false alarms. Furthermore, usually only rapid movements will trigger an alarm. As a consequence there is the danger that large insects trapped in the vehicle interior can trigger the alarm by their movement. The continuous generation of sound waves likewise results in a high energy consumption.